The 2004 client represents a lost innocence: a time when Roblox wasn't a multi-billion dollar corporation, but two guys in a California office trying to make blocks stick together over a dial-up connection.
client (then known as DynaBlocks ) is extremely rare because the game was in a closed, private beta state with very few users. While a fully authentic 2004 installer is not publicly available for general use, the "Roblox revival" community has archived several early versions and recreations. Roblox Support Available Archives & Recreations roblox 2004 client install